FIELD OF THE INVENTION
The present invention relates to a metal powder pigment, its preparation and its use in coating systems to provide novel visual effects.
Metal powder may be prepared in an atomizer (see for example U.S. Pat. No. 4,705,560). Conventional atomized powder produced in this way consists essentially of fairly uniform particles having a median particle size (D.sub.50) of 1 to 300 .mu.m, usually 3 to 75 .mu.m, more usually 5 to 40 .mu.m and an aspect ratio, that is ratio of largest dimension to smallest dimension, of about 1.5 to 5/1. By modification of the atomization process it is possible to reduce the aspect ratio towards unity, i.e. towards a perfect sphere. In practice, such so called spherical atomized powders are of similar diameters to the foregoing, but have aspect ratios of at most 1.5:1, more usually less than 1.3:1. By virtue of their sphericity, the sphere being the geometrical shape having the lowest ratio of surface area to volume, atomized metal powder particles have poor covering power and are dull and relatively non-reflective.
Metal flake pigment may be prepared from atomized metal powder by either wet or dry ball milling. In wet ball milling (see for example U.S. Pat. No. 1,569,484 and U.S. Pat. No. 3,901,688) metal powder is ball milled with an organic liquid such as mineral spirits and a small amount of a lubricant such as oleic acid or stearic acid, and the resulting flakes are separated, for example by wet sieving, to provide the desired particle size distribution and brought to a paste-like consistency of, typically, 55 to 80% by weight. In dry ball milling (see for example U.S. Pat. No. 4,115,107) metal powder is ball milled in the absence of liquid but inert gas is passed through the mill at such a rate that flake is removed as it is formed.
In these milling processes flakes are formed substantially instantaneously whenever an essentially spherical powder particle is trapped between milling media, usually polished metal balls which have a very high mass relative to that of the trapped particle. Thus even after only a short milling time, a sample removed from the mill base in a wet milling process will contain flakes in addition to the starting powder particles (which have not been trapped between the milling media).
The metal flakes produced in this way have a significantly different geometry to that of the starting, essentially spherical, powder particles. Commercially available metal flakes typically have a second largest dimension, measured at right angles to the largest dimension, of less than 150 .mu.m, more usually 8 to 35 .mu.m, and an average thickness, measured at right angles to the other two dimensions, of only 0.02 to 2 .mu.m, more usually 0.05 to 1.5 .mu.m, indeed generally less than 1 .mu.m and often only about 0.1 .mu.m. The metal flakes thus necessarily have a large aspect ratio, very rarely less than 10/1 and more commonly in a range from 15/1 to over 100/1. Thus lenticular flakes generally have aspect ratios of 25/1 to 100/1, while "cornflakes" may have aspect ratios of 100/1 and greater.
The use of metal flake pigments, especially aluminum or aluminum alloy flake pigments, in surface coating systems, such as automotive paints, is well known. As a consequence of their shape, metal flake pigments provide a visual effect in surface coatings, known as "flip" or "flop", wherein the colour depth of the coating alters according to the angle of viewing. A description of the origin of this effect and its measurement will be found in U.S. Pat. No. 4,590,235 and in EP-A-422357. Metal flake pigments are usually incorporated in surface coating systems at concentrations of 1 to 30% by weight, depending upon the intended application. A particularly important application for metal flake pigments, principally aluminum or aluminum alloy flake pigments, is in automotive paints and for this application it is well known to employ a two-coat system, wherein the flake pigment is laid down in a first coat and subsequently overcoated with a clear top coat. This so called base coat-clear system provides an improved appearance and resistance to weathering.
In recent years, there has developed an increasing consumer demand for novel visual effects in automotive coatings. This has resulted in the introduction of pearlescent or mica pigments (as described in U.S. Pat. No. 4,956,019); in surface colored aluminum flakes (as described in EP-A-33457) and in multicoat systems wherein mica and/or metal flakes are used with organic or inorganic pigments in two or more coating layers, chiefly to provide dichromatic hue shift effects. A typical example of such a multicoat system is described in EP-A-388931.
It is against this background of a constant search for coating systems possessing novel or unique color effects that the present invention has been developed.